The scanning electron microscope (SEM) that allows observation of a microstructure has been employed for observation of samples in various fields. Especially, the low-acceleration type SEM that applies the electron beam to the sample at the acceleration of 5 kV or lower is very useful as it provides such advantages of causing less damage to the sample, having small penetration depth of the electron beam to allow observation of the nano-structure of the surface, and allowing easy observation of the sample that is likely to be charged under the condition with well balance between quantities of the incident electron and the electron emitted from the sample.
Under the aforementioned low-acceleration condition, chromatic aberration of the electron beam on the objective lens is increased to deteriorate the resolution. Accordingly, high-resolution observation requires the technology for suppression of the chromatic aberration. One of technologies is so called a semi-in-lens or a snorkel lens formed by designing the magnetic path of the objective lens so that the peak of the magnetic field on the axis is closer to the sample side than the side of the objective lens. This reduces a focal point distance f that substantially contributes to the chromatic aberration, resulting in the high resolution even at the low acceleration condition.
Another technology is the retarding method that generates an electric field between the sample and the objective lens, which decelerates the electron beam of the probe (for example, Patent Literature 1 and 2). Normally, the negative voltage is applied to the sample. In this case, even if the incident energy to the sample is reduced, the energy passing through the objective lens can be intensified, thus reducing the chromatic aberration of the objective lens. Employment of both the semi-in-lens and the retarding technology realizes the high-resolution observation irrespective of the low incident energy. Upon discrimination of the electron energy generated from the sample under the aforementioned conditions, irregularities on the surface are identified by the secondary electron (SE) having low energy. Meanwhile, the contrast that reflects the atomic number and density of the material is obtained through measurement of the backscattered electron (BSE) having high energy. The aforementioned information may be derived from an SEM image, which allows the analysis in a short time.
In the case where the retarding method is employed, if the incident energy is 1 kV, the difference in generated energy between the SE and BSE becomes 1 kV at most. When the energy of 2 kV is applied for retarding, each orbit of both electrons becomes substantially the same. It is therefore difficult to perform energy discrimination and detection. For this, another lens or ExB field (the state where the electric field orthogonally intersects the magnetic field) for detection is provided at the position closer to the side of the electron source than the objective lens to perform the energy discrimination using the phenomenon that the orbit varies in accordance with the energy difference as disclosed in Patent Literature 1.
When intending to have the observation by changing the incident energy to the sample, the user changes the acceleration voltage of the probe electron beam. In such a case, the variable conditions for the electronic optical system have to be adjusted. Much time and high operation skill are required to change the focal point, astigmatism, visual field, and contrast and brightness of the image. An idea to provide the function for automatically adjusting those conditions may be easily thought. However, it is difficult to establish such automatization in conformance to all conceivable cases. As a result, the function is effective under the limited condition, which also adds cost for the apparatus, resulting in high price.